The invention relates to a charge-coupled imaging device having a 3-phase charge-coupled device comprising a row of clock electrodes situated above a charge transport channel defined in a semiconductor body, clock voltage means being present for applying clock voltages to the clock electrodes. The invention also relates to a camera provided with such an imaging device. Although the invention will be described below in particular with reference to two-dimensional imaging devices, also called image sensors, it should be borne in mind that the invention may also be advantageously applied to one-dimensional imaging devices or line sensors.
A device of the kind described above is known inter alia from the publication "Interlacing in Charge-Coupled Imaging Devices" by C. H. Sequin, published in IEEE Transactions On Electron Devices, vol. ED-20, No. 6, June 1973, pp. 535-541. This describes a device which is operated in the so-called interlace mode whereby such clock voltages are applied to the clock electrodes that in two consecutive integration periods rasters of picture elements are defined which are shifted relative to one another over a distance of half a pitch between the picture elements (interlacing).
The principle of forming two consecutive rasters which are mutually shifted, often called "interlacing" in the literature, corresponds to the known manner in which a TV picture screen is scanned in two consecutive half rasters. In general, interlacing is obtained in charge-coupled image sensors by means of the applied clock voltages which determine the potential profile in the charge transport channel, and thus the positions of the consecutive picture elements. It is possible to shift the (centers of gravity of the) picture elements over half a pitch, or at least over substantially this distance, in that the voltages are changed during the second integration period compared with the voltages which were applied during the first integration period. This method has the advantage that the number of lines is fictitiously doubled without an accompanying requirement to double the chip surface area used.
In general, the size of the picture elements, also called pixels, is determined by the number of phases by which the device is operated. A frequently used embodiment comprises a 4-phase charge-coupled device in which one picture element corresponds to four clock electrodes. During operation, for example, a blocking voltage is applied to one of these four electrodes, so that a potential barrier against information-bearing charge carriers is induced in the subjacent portion of the charge transport channel. Voltages of the active level are provided to the other three electrodes, a potential well being formed in the channel, in which well a generated charge is stored. The boundaries between the consecutive pixels are situated at least substantially in the centers of the potential barriers. Interlacing may be obtained simply in that the pattern of voltages applied for the first raster is shifted over a distance of half a pixel, i.e. two electrodes, for the second raster.
Besides 4-phase CCDs, 3-phase CCDs are also generally known for various applications. Compared with 4-phase CCDs they have the advantage that only three clock electrodes instead of four are necessary for each charge package. When used in an imaging device this results in a much smaller chip surface area and/or a better resolution. A problem with 3-phase CCDs in imaging devices, however, occurs when the device must be capable of operation in the interlace mode, since it is not possible with a pixel size of three electrodes to shift the pattern of applied voltages over a distance of half a pixel in the simple manner described above.
The publication by Sequin cited above describes a 3-phase CCD in which the first raster is integrated below the electrodes of phase 1 only, while the electrodes of phases 2 and 3 are at the blocking voltage level. For the second raster the situation is reversed, the electrodes of phases 2 and 3 being integrating whereas the electrodes of phase 1 are blocking. It can be easily ascertained that the centers (of gravity) of the pixels of the two rasters are at a distance of approximately 1.5 electrodes, i.e. half a pixel, as is required for interlacing. A disadvantage of this known interlacing is that the conditions under which the two rasters are formed are so substantially different that the transition from the one raster to the other raster, all other conditions being equal, is visible in the display of the image (flicker). This additional noise is particularly unpleasant in versions with vertical anti-blooming, in which the substrate, which is separated from the charge transport channel by a thin layer of the opposite conductivity type to the substrate, which layer is fully depleted during operation, forms the drain zone for an excess of charge carders in the case of overexposure. Such a device is described in, for example, U.S. Pat. No. 4,654,682. Since only one electrode is in the blocking state in the one raster, whereas the other raster has two blocking electrodes, first of all the sensitivity will change, not exclusively but certainly particularly in the versions having vertical anti-blooming as described above in which a relatively greater portion of the charge generated below the blocking electrodes will be drained off to the substrate in the case of two blocking electrodes than in the case of one blocking electrode. For the same reason, the dark currents will also be different in the two rasters.